The Dutch National Institute for Public Health and the Environment (RIVM) said Sunday that "very low" levels of man-made radioactivity were found over the three countries. There was no impact on the environment or human health, it said.
"The combination of radionuclides may be explained by an anomaly in the fuel elements of a nuclear power plant," RIVM suggested after performing a calculation to find the source of the radionuclides, which are atoms with an unstable core.
"The calculations indicate that the nuclides come from the direction of western Russia. Determining a more specific source location is not possible with the limited data available," RIVM said on its website. It made clear that "no specific country of origin can be pointed out at this moment".
In response, Russia stated that no incidents were recorded at two plants in the west of the country.
"No incidents were recorded at the Leningrad nuclear power plant and the Kola nuclear power plant, both stations operate normally, there have been no complaints about the equipment's functionality," said a statement reported by state media outlet RIA Novosti from Rosenergoatom, part of the Rosatom state nuclear energy corporation that oversees all of Russia's nuclear infrastructure.
Kremlin spokesperson Dmitry Peskov also denied any incident had occurred.
"We have an exceptional and modern system of monitoring nuclear safety and as you've seen there were no alarms related to any threatening or emergency situations," Peskov said Monday. "We don't know what the source is for these reports of specialists in the Netherlands," he added.
A communications officer at Finland's Radiation and Nuclear Safety Authority (STUK) told CNN on Monday that Finnish authorities were not speculating on the exact source but have indications that it came from a nuclear reactor.
"STUK has been in contact with domestic nuclear power plants. They have not detected substances detected by STUK in their own emission monitoring. It is therefore unlikely that the radioactive substances detected would be of domestic origin," the authority said in a press release.
A representative for the Swedish Radiation Safety Authority told CNN that "very low levels" of Cs-134, Cs-137, Ru-103 and Co-60 isotopes were found in two places: Visby, between June 8-15, and Stockholm, between June 22-23. The representative did not speculate on the origin and location of the source of the material.
A press officer at the Norwegian Radiation and Nuclear Safety Authority told CNN that checks were carried out around two weeks ago, before their Nordic counterparts. The level detected was also said to be very low and they are carrying out further measurements this week.